1. Field of the Invention
The present invention relates to extrudable lanthanum manganite pastes, dried unfired extruded lanthanum manganite bodies, porous lanthanum manganite sintered bodies (e.g. tubes), and methods of manufacturing the same.
2. Description of the Prior Art
Lanthanum mangarite has recently become of increasing interest as a temperature resistant electrically conductive ceramic material. In the art, the term lanthanum manganite includes doped lanthanum mangarites. For this specification and claims we define lanthanum marganite as an oxide of perovskite crystal structure having the formula EQU La.sub.1-x A.sub.x Mn.sub.1-y B.sub.y O.sub.3 .+-..delta.
wherein
0.ltoreq.x.ltoreq.0.75 PA1 0.ltoreq.y.ltoreq.0.5 PA1 0.ltoreq..delta..ltoreq.0.2
and A is one or more of alkaline earth metals (particularly Ca and Sr), Y and rare earth metals, and B is one or more of Cr, Ni, Co, Al and Zn. A lanthanum manganite sintered body is typically made by mixing starting components e.g. oxides, calcining the mixture to generate the lanthanum mianganite, grinding the calcined product to suitable powder size, shaping the powder and firing.
One use of lanthanum manganite sintered bodies is in solid oxide fuel cells ("SOFs"). SOFCs are promising power generation systems having high efficiency and cleanliness. SOFCs are composed of air electrodes, fuel electrodes, solid electrolytes and interconnectors. For the air electrodes, porous sintered lanthanum manganite having high electroconductivity is used. There are two types of lanthanum manganite air electrodes; one is a thin film electrode on a porous support tube made of, for example, calcia-stabilized zirconia, and the other is a porous lanthanun manganite tube electrode which itself is a support, The latter type is preferable because of simplicity of cell manufacturing process, which leads to low manufacturing cost. This type of air electrode is made by processes such as extrusion, slurry casting, injection molding and so on. Extrusion is especially convenient. After extrusion of a tube, an end closure of the tube may be added, if desired.
However, in a drying process after extrusion, air electrode tubes often microscopically crack and, after sintering, the tubes may be severely cracked reducing production yield. The reason why such cracks occur is not clear as yet.